One conventional form of a DC arc furnace has at least one arcing electrode and beneath it a hearth for containing a melt. DC power applied with one pole connected to the melt and the other to the electrode forms an arc putting heat into the melt. When the electrode is of the consumable type, smoother arc operation and less electrode wear is obtained when the connections make the melt anodic and the electrode cathodic.
For the electrical connection with the melt, it has been considered safer to provide a melt connection horizontally offset from beneath the arc, an arrangement reducing the risk of a melt breakout. This has the disadvantage that the arc is deflected either towards or away from the melt electrode, in either case resulting in an arc flare that is directed steadily against a localized portion of the furnace side wall lining with consequent rapid erosion of the lining at that portion.
A hearth made of carbon bricks or blocks can contain the melt and form an electrical connection distributing the electromotive forces symmetrically around the electrode so that the arc is vertical with its arc flare distributed uniformly around the furnace side wall lining, avoiding the concentrated erosion effect. However, such an electrically conductive hearth is not only expensive to build, but it also is expensive to maintain in serviceable condition as contrasted to what is involved by a conventional hearth with which a side melt contact can be associated.
A conventional electric arc furnace hearth has a steel shell extending under the furnace bottom and supporting a layer of refractory bricks, such as silica or magnesite bricks, and above which a layer of quartzite, dolomite, or aluminum or magnesium oxide particles forming a refractory compound is rammed. This rammed compound is initially moist and plastic and is hardened by heating prior to the hearth receiving a melt. This is a well-known hearth construction and has a reputation for safety and relatively low cost maintenance. The rammed refractory is relatively non-conductive in the sense that although when dried and contacted by the melt its conductivity may increase somewhat, the conductivity is not nearly adequate to permit the hearth to be used as a melt connection.
Some refractories capable of containing the melt do become conductive at the temperature of the melt, but suffer the disadvantage that they must be brought to that temperature before an arc furnace can be put into operation with the hearth forming a melt connection, and this is an operational disadvantage. To provide a melt connection permitting the maintenance of a vertical arc, suggestions have been made to use vertical plate electrodes positioned centrally in the furnace bottom with the relatively non-conductive refractory compound between the plates. With the plates directly below the arc, such suggestions have been considered as involving too great a risk of melt breakout through the hearth.